One additional note to Wayne's posting:
If an on-air signal sounds bad it is frequently because the user is over-driving
the amplifier, using and adjusting the amplifier to Rig ALC incorrectly (we
strongly recommend not to use ALC) or possibly has gone inside the amp and
manually mis-adjusted the fixed bias to the PAs.
73,
Eric
elecraft.com
On 1/7/2015 3:35 PM, Wayne Burdick wrote:
Recently there was speculation about the KPA500 using an "Electronic Bias
System" (EBS), and whether that might be implicated in a noise issue.
First, we have never heard any reports of a "bias crunch" or "paper crunch"
problem with the KPA500. This particular noise is quite distinctive, which easily differentiates
itself from other common station integration problems such as overdrive or RFI.
Second, there is no "mod" available, because the KPA500 does not now, nor ever
has had an EBS circuit.
Details:
The KPA500 definitely does not suffer from so-called "bias crunch"--a noise at
the end of a SSB transmission similar to that made by wrinkling a sheet of paper--because
full bias is applied immediately whenever the KPA500 is in OPER mode and is keyed. Full
bias is maintained until the KPA500 is unkeyed.
EBS is often used with vacuum tube amplifiers that require large bias currents
to meet their linearity requirements. In some cases, the bias current required
results in a standing power dissipation between 1/2 and 2/3 of the plate
dissipation rating of the tube. Thus, the tube generates more waste heat while
idling than under key down conditions. EBS works by sampling the RF drive
signal. Until the drive reaches a watt or two, the tube is either cut off or
idling with extremely low current. Then, after the input power threshold is
reached, the EBS suddenly applies full bias. Just like a receiver's AGC attack
and decay times, EBS timing is tricky and critical to proper operation without
causing unwanted artifacts. The paper crunch noise occurs when the tube is
caught amplifying without bias (temporarily operating in non-linear Class-C
mode) due to improper time constants.
When the KPA500 was designed, it was determined that the bias current required
for proper linearity was low enough that EBS is not needed. As any KPA500 owner
can quickly verify by placing the amplifier in OPER mode and grounding the KEY
IN line, the front panel meter reads this standing bias current--even with zero
RF drive. Amplifiers that have EBS do not show any standing bias current
without drive.
Any KPA500 that generates a paper-crunch artifact is not operating correctly.
(I'd like to thank Bob, K6XX, for researching EBS so we could provide the above
information.)
73,
Wayne
N6KR
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